The forensic detection and analysis of surface residues is one of the most important tools used by forensic experts for gathering evidence in crime scene investigations. For linking one or more suspects to a crime scene, fingerprint analysis of the crime scene is of particular importance in such investigations.
Fingerprints found on surfaces can be categorized according to three main types: patent (visible), plastic, and latent. Patent fingerprints result from the transfer of a visible material (e.g., paint, ink, blood, etc.) from the surface of a finger to another surface. The transferred material renders a patent fingerprint readily viewable without the use of imaging enhancement techniques. A plastic fingerprint is a fingerprint impression made in a deformable material, such as wax, soap, or putty. As used herein, a latent fingerprint is one which is not readily visible to the naked eye. Since latent fingerprints are not visible to the naked eye, an imaging enhancement technique is required to make them viewable.
The latent fingerprint is formed mainly by secretions emitted from the fingers, i.e., from sweat, and typically some portion of dirt, microorganisms, and oils. The secretion can be classified as either eccrine (i.e., “clean” and low in oils) or sebaceous (i.e., “dirty” and copious in oils).
The imaging (i.e., detection) of latent fingerprints remains among the most challenging. This is particularly so for the reason that a latent fingerprint can only become visible by application of an imaging enhancement technique. However, during the time period between when the latent fingerprint was originally deposited (i.e., as a fresh latent fingerprint) and the time of imaging, the latent fingerprint often has ample time to decompose. There are several modes of decomposition, all of which work to obscure the fingerprint and make it more difficult for imaging techniques to elucidate the fingerprint. The time period for significant amounts of decomposition to take place may in some cases be only a few hours. Some modes of decomposition include thermal, photonic, and chemical degradative processes. For example, fingerprints deposited on many surfaces often go undetected once the latent prints age over a few hours, especially when exposed to UV radiation (e.g., from sunlight or fluorescent lighting).
There are several other factors that can make the process of imaging latent fingerprints even more challenging. For example, the imageability of latent fingerprints by current techniques is very much dependent upon the surface on which the fingerprint is found. In particular, using current techniques, latent fingerprints on skin (e.g., on a corpse) are particularly difficult, if not impossible in most cases, to discern Surfaces containing iron (III), such as steel, also rapidly decompose latent fingerprints. In addition, latent fingerprints differ in their imageability based on their chemical composition (e.g., eccrine or sebaceous). Eccrine prints, as found more predominantly from children (particularly pre-pubescent), are generally more difficult to image.
Numerous techniques are known for the detection or analysis of latent fingerprints. For example, silver nitrate has been used to develop latent fingerprints by its reaction with salts contained in the fingerprint and subsequent exposure to an actinic light source. However, exposure of the fingerprint to moisture severely limits utility of this method.
The ninhydrin technique makes use of the reaction between amino acids found in a fingerprint with triketohydrinden hydrate to form a visible fingerprint image. However, it is well known that not all fingerprints contain a suitable level of amino acids to make the ninhydrin technique generally effective.
The fingerprint dusting method involves depositing a visible powder on a surface suspected of containing latent prints. The powder adheres to oils in the print to make the print visible. However, latent prints that are not oily are generally not amenable to this method. In addition, the efficacy of the technique is very dependent on the technical proficiency of the operator.
In the iodine technique, iodine crystals are warmed in the vicinity of a surface suspected of containing latent prints. The resulting iodine vapor reacts with lipids in the latent print which causes the print to become visible. Similarly to the powder method, the iodine method is generally useful only for oily prints. In addition, the deposited iodine quickly fades over time to eventually leave the original invisible print. The iodine is also strongly oxidizing, which can cause damage to the surface or adversely alter the residue.
In the fluorogenic visualization of latent prints, the latent print is treated with one or more chemical reagents (e.g., a luminescent dye) that react with compounds in the print to form a fluorescent product. However, the number of active compounds in the print capable of forming a fluorescent product are limited. For example, the technique typically relies on the fingerprint containing certain amino acids.
In the cyanoacrylate (superglue) fuming technique, cyanoacrylate monomer (e.g., as obtained by heating a superglue composition) reacts with a one or more water-soluble components to cause polymerization of the monomer. Some of the water-soluble components that may initiate polymerization include, for example, sodium lactate, inorganic salts, free amino acids, urea, mucoproteins, and ammonia. Sebaceous components are generally inert in the initiation process, but can act to solubilize and accumulate the monomer for subsequent polymerization. In order for the cyanoacrylate method to work, the print needs to be hydrated. However, unlike oily prints, clean (eccrine) prints do not contain hygroscopic materials such as di- and mono-acyl glycerols and glycerol. As a result, clean prints are not able to maintain a hydrated print composition, and thus, become dehydrated within relatively short time frames to an extent that the superglue fuming technique is no longer effective. For example, clean prints that are older than 48 hours prior to fuming are typically so severely degraded that the superglue fuming technique is no longer useful. Attempts at simple rehydration of the prints have generally not been successful. Furthermore, latent fingerprints lose cyanoacrylate initiator (particularly lactate) via photodegradation. Therefore, latent fingerprints that have undergone photodegradation are also difficult if not impossible to image using the superglue fuming method.
Optical vibrational spectroscopic imaging work has been conducted on latent fingerprints using Fourier transform infrared (FTIR) spectroscopy. However, the FTIR technique generally suffers from a high amount of interference from water background signals. In addition, the FTIR technique is mainly applicable to oily prints, and is significantly limited when applied to clean prints. The FTIR technique is also limited in its effectiveness for imaging decomposed latent prints and latent prints residing on difficult surfaces, such as skin.
As shown, there remains a need in the art for an effective and reliable method for the imaging of a wide range of latent fingerprints under a variety of conditions. There is a particular need for a method capable of imaging latent fingerprints that are traditionally difficult or impossible to image using methods known in the art, particularly latent fingerprints that have undergone decomposition or that reside on difficult surfaces, such as skin, steel, and rough or porous surfaces. There is an additional need in the art for a method which, during the course of imaging a fingerprint or other type of print, is capable of detecting or identifying one or more chemical species of interest in the fingerprint or other print.